Conventionally, as a method for improving the corrosion resistance of aluminum and aluminum alloys (hereinafter referred to as aluminum-based materials), an anodizing treatment has been employed in which a porous anodic oxide coating is formed on a surface of an aluminum-based material. In the porous layer of the anodic oxide coating, which is formed mainly by direct-current electrolysis, pores are regularly arranged in general, although this greatly depends on the electrolysis conditions. For this reason, its porosity is one of the causes of degradation of the corrosion resistance. To improve the corrosion resistance of an anodic oxide coating, a sealing treatment for filling the pores or the like is conducted after the anodizing treatment.
As such an anodizing treatment, a method has been disclosed in which a first anodized film is formed on a surface of an aluminum-based material by a first current application, and then a second anodized film is formed by a second current application with a larger quantity of electricity than the quantity of electricity in the first current application, so that the flatness and smoothness are increased (PTL 1). In addition, a method has been disclosed in which an anodic oxide coating having an increased corrosion resistance is formed by using a phosphoric acid electrolytic liquid, employing a voltage to 20 V or lower at an initial stage, and then increasing the treatment voltage up to a final voltage which is five times or more the treatment voltage at the initial stage (PTL 2).
In addition, a method has been disclosed in which an anodic oxide coating with a high porosity and a large film thickness is formed to lower the thermal conductivity and to improve the fuel-efficiency (PTL 3). A technology has been disclosed in which an anodic oxide coating provided with a bonding region where each hollow cell forming an anodic oxide coating is bonded to adjacent hollow cells and a nonbonding region where three or more adjacent hollow cells are not bonded to each other is formed by application of direct-current electrolysis, and then pores are filled by a treatment with boiling water or steam or by coating with a thin film (PTL 4). Moreover, a technology has been disclosed by which a dense anodic oxide coating is formed in the vicinity of an aluminum-based material by repeating a step of applying a positive voltage and a step of removing electric charge by using a power source having an alternating current component (PTL 5).